2. Field of the Invention
The present invention relates to a pressure sensor which is capable of detecting the pressure difference from a reference atmosphere in air or in liquid in terms of an electric signal output.
2. Description of the Prior Art
Conventionally, pressure measurement by use of mechanical changes of a Bourdon's tube, bellows, diaphragm or the like has been performed for measurement in the atmosphere or liquid. It is generally used extensively because of its low cost and simplicity. With the development of electronics technology, the development of pressure sensor is desired which measures pressure changes as an electric signal output in order to interact readily with electronic devices. The pressure sensor which measures pressure changes in terms of an electric signal output is capable of being easily connected with a data processing system and is simple in automatic measurement and control. Also, outputting of measurement values directly as electric signals enables measurement of pressures with high accuracy, provides quicker response speed, and makes it easier to provide a sensor of smaller size and lighter weight.
Accordingly, various types of pressure sensors have been studied and developed, as listed below:
(1) Pressure sensor with a metal foil strain gauge being mounted on a metal diaphragm. PA0 (2) Silicon diaphragm pressure sensor PA0 (3) Pressure sensor using piezoelectric material such as PVDF or ZnO. PA0 (4) Quartz pressure sensor PA0 (5) Pressure sensor using capacitance changes
In such conventional pressure sensors as described above, the pressure sensor (1) with a metal foil strain gauge mounted on a metal diaphragm uses the changes in the electric resistance of the metal foil through the strain of the metal foil through deformation of the diaphragm by pressure. The advantage is that high pressures may be measured, also the temperature characteristics and the material property are superior. However, the disadvantage is that the sensitivity is poor and reduction in size and weight is difficult.
The silicon diaphragm pressure sensor (2) uses the piezoresistance effect in which the specific resistance of Si changes when pressure is applied upon the Si crystal. This pressure sensor, which uses Si as a material, may be mass-produced and is likely to be integrated with peripheral circuits by the use of semiconductor technology, while a temperature compensation circuit is required, because it depends largely upon temperature. A pressure sensor with a temperature compensation circuit formed integral with the Si pressure sensor on the same Si substrate is manufactured. But the pressure sensor of this type has disadvantages in that it is expensive and that the sensor element can be easily damaged due to the poor mechanical strength of the Si diaphragm.
The pressure sensor (3) using piezoelectric material such as PVDF or ZnO is a pressure sensor using the piezoelectric effect in which piezoelectric materials generate electromotive force when deformed by pressure. The advantage is that the pressure sensor is small in size and light in weight and that its output is large. The disadvantage is that the detection accuracy is poor and that noise caused by vibrations are likely to be picked up.
The quartz pressure sensor (4) uses the property of quartz whose oscillation frequency linearly changes with pressure. The disadvantage is that it is expensive, and reduction in size and weight is difficult.
The pressure sensor (5) detects the movement of a diaphragm an electrostatic capacitance variation. Recently, an ultra-small electrostatic capacitance variation type pressure sensor using an Si diaphragm was developed. It is pointed out that the ultra-small electrostatic capacitance variation type pressure sensor is more sensitive and stable than an Si pressure sensor using the piezoresistance effect. The disadvantage is that the ultra-small sensor has an extremely small value of electrostatic capacitance, i.e., the impedance is extremely high and is likely to be influenced by external noise.
As described hereinabove, the conventional pressure sensors are not sufficient in performance or price and have various problems that have to be solved before being put into practical use.
Two of the present inventors have proposed a novel and useful pressure sensor as U.S. patent application Ser. No. 838,838, filed on Mar. 12, 1986, and assigned to the same assignee, which may be manufactured in ultra-small size and at low cost by semiconductor technology through the use of a field effect transistor.
In the field effect pressure sensor proposed previously by the present assignee, a hollow chamber is provided in the upper portion of the gate insulating film of the field effect transistor, and the gate electrode, which may be movably deformed by pressure, is formed on the gate insulating film through the hollow chamber. The gate electrode provided through the hollow chamber on the gate insulating film is movably deformed by pressure so that the distance between the gate electrode and the gate insulating film changes to cause the electrode field intensity applied to the channel to vary. As a result, the pressure is detected in terms of the drain current variation of the field effect transistor.
In the field effect pressure sensor previously proposed by the present applicant, the gate electrode provided through the hollow chamber on the gate insulation film as described hereinabove is movably deformed by pressure so that the distance between the gate electrode and the gate insulating film changes to cause the field intensity applied upon the channel to vary. Consequently, the pressure is adapted to be detected in terms of the drain current variation of the field effect transistor. After various examinations, the output variation through temperature variation and aging variation is comparatively large.
Namely, in the field effect pressure sensor previously proposed by the present applicant, an absolute pressure type of pressure sensor is constructed which is adapted to keep the hollow chamber under a constant pressure or vacuum.
The absolute pressure type of field effect pressure sensor retains the hollow chamber under constant pressure or vacuum to use the movable deformation of the metallic film diaphragm on the upper portion of the gate insulating film through pressure variation of the outer atmosphere. In the absolute pressure type of pressure sensor, the hollow chamber is required to be normally kept at constant pressure or in a vacuum independently of variation aging to cause the hollow chamber to be completely air-tight. The spacer material for forming the hollow chamber has to be strictly selected, and the adherence property between the spacer and the metallic film diaphragm is required to be made sufficiently higher.
Furthermore, when the closed hollow chamber is constructed to be set under a constant pressure, the pressure changes through the temperature of the hollow chamber are large, and the metallic diaphragm is displaced because of temperature changes to cause the output of the field effect transistor to vary.